Raman study of the phase transitions sequence in pure WO₃ at high temperature and in H_xWO₃ with variable hydrogen content

An extensive investigation of the temperature dependence of Raman spectra has been carried out on WO₃ powders from room temperature to 800 °C. In particular the orthorhombic-to-tetragonal phase transition occurring at about 740 °C has been studied for the first time. The Raman active mode at 710 cm^-1 of the orthorhombic phase disappears from the spectrum at temperature below the phase transition point and the Raman activity in the tetragonal phase results very low. A comparative study of hydrogenated tungsten bronzes H_xWO₃ (x≤0.23), where the same transition sequence is driven by an increase of the proton concentration from x = 0 to 0.23, reveals similar behaviour of the high frequency Raman active modes, which are better resolved due to the weaker anharmonic interactions.